Jitter has been defined as the short-term non-cumulative variations of the significant instants of a clock or data signal from their ideal positions in time. The significant instants can be the rising or falling edge of a pulse. Jitter has a variety of sources including imperfections in oscillators, thermal noise, instabilities in the oscillator electronics, RF interference and the transmission of signals through communications equipment such as regenerative repeaters. Large amounts of jitter can cause equipment to fail; hence, designers must look to jitter measurement techniques and devices to properly address the effects of jitter.
Two basic methods may be employed to measure jitter and its effects on equipment: the oscilloscope method and the phase detector method. According to the oscilloscope method a first clock source provides a jitter-free trigger signal for an oscilloscope and the first clock's frequency reference is locked to a second clock source. The second clock source is modulated by jitter and input to a pattern generator which provides jittered data to equipment being tested. The input and output waveforms to the equipment may then be analyzed with oscilloscope functions. Also, the oscilloscope may be used to view eye patterns of signals. The oscilloscope method, however, is limited to jitter having amplitudes less than one unit interval (one unit period).
According to the phase detector method, the phase of the recovered clock, output from the equipment receiving a data signal having jitter, is compared with the phase of the jitter-free clock source. The resulting phase difference is the jitter amplitude measured in unit intervals. A frequency divider placed between the recovered clock signal and the phase detector extends the range of phase detector beyond the unit interval.
Accurately measuring jitter has become increasingly important in testing communications components that operate at high bit rates. As bit rates increase with developing fiber optic technology, jitter increases causing communication components to malfunction. Thus, accurate jitter measurements are necessary for designers to account for jitter. However, measuring the jitter of a high bit rate signal requires the use of costly sampling hardware (e.g., gallium arsenide semiconductor technology).